Thermodynamics, kinetics, and mechanism of the stepwise dissociation and formation of Tris(L-lysinehydroxamato)iron(III) in aqueous acid

Estradiol-Based Salicylaldehyde (Thio)Semicarbazones and Their Copper Complexes with Anticancer, Antibacterial and Antioxidant Activities

A series of novel estradiol-based salicylaldehyde (thio)semicarbazones ((T)SCs) bearing (O,N,S) and (O,N,O) donor sets and their Cu(II) complexes were developed and characterized in detail by ¹H and ¹³C nuclear magnetic resonance spectroscopy, UV-visible and electron paramagnetic resonance spectroscopy, electrospray ionization mass spectrometry and elemental analysis. The structure of the Cu(II)-estradiol-semicarbazone complex was revealed by X-ray crystallography. Proton dissociation constants of the ligands and stability constants of the metal complexes were determined in 30% (v/v) DMSO/H₂O. Estradiol-(T)SCs form mono-ligand complexes with Cu(II) ions and exhibit high stability with the exception of estradiol-SC. The Cu(II) complexes of estradiol-TSC and its N,N-dimethyl derivative displayed the highest cytotoxicity among the tested compounds in MCF-7, MCF-7 KCR, DU-145, and A549 cancer cells. The complexes do not damage DNA according to both in vitro cell-free and cellular assays. All the Cu(II)-TSC complexes revealed significant activity against the Gram-positive Staphylococcus aureus bacteria strain. Estradiol-TSCs showed efficient antioxidant activity, which was decreased by complexation with Cu(II) ions. The exchange of estrone moiety to estradiol did not result in significant changes to physico-chemical and biological properties.

Interaction of Triapine and related thiosemicarbazones with iron(III)/(II) and gallium(III): a comparative solution equilibrium study

Stoichiometry and stability of Ga(III), Fe(III), Fe(II) complexes of Triapine and five related α-N heterocyclic thiosemicarbazones with potential antitumor activity have been determined by pH-potentiometry, UV-vis spectrophotometry, (1)H NMR spectroscopy, and spectrofluorimetry in aqueous solution (with 30% DMSO), together with the characterization of the proton dissociation processes. Additionally, the redox properties of the iron complexes were studied by cyclic voltammetry at various pH values. Formation of high stability bis-ligand complexes was found in all cases, which are predominant at physiological pH with Fe(III)/Fe(II), whilst only at the acidic pH range with Ga(III). The results show that among the thiosemicarbazones with various substituents the N-terminal dimethylation does not exert a measurable effect on the redox potential, but has the highest impact on the stability of the complexes as well as the cytotoxicity, especially in the absence of a pyridine-NH(2) group in the molecule. In addition the fluorescence properties of the ligands in aqueous solution and their changes caused by Ga(III) were studied.

Siderophores are not involved in Fe(III) solubilization during anaerobic Fe(III) respiration by Shewanella oneidensis MR-1

Shewanella oneidensis MR-1 respires a wide range of anaerobic electron acceptors, including sparingly soluble Fe(III) oxides. In the present study, S. oneidensis was found to produce Fe(III)-solubilizing organic ligands during anaerobic Fe(III) oxide respiration, a respiratory strategy postulated to destabilize Fe(III) and produce more readily reducible soluble organic Fe(III). In-frame gene deletion mutagenesis, siderophore detection assays, and voltammetric techniques were combined to determine (i) if the Fe(III)-solubilizing organic ligands produced by S. oneidensis during anaerobic Fe(III) oxide respiration were synthesized via siderophore biosynthesis systems and (ii) if the Fe(III)-siderophore reductase was required for respiration of soluble organic Fe(III) as an anaerobic electron acceptor. Genes predicted to encode the siderophore (hydroxamate) biosynthesis system (SO3030 to SO3032), the Fe(III)-hydroxamate receptor (SO3033), and the Fe(III)-hydroxamate reductase (SO3034) were identified in the S. oneidensis genome, and corresponding in-frame gene deletion mutants were constructed. DeltaSO3031 was unable to synthesize siderophores or produce soluble organic Fe(III) during aerobic respiration yet retained the ability to solubilize and respire Fe(III) at wild-type rates during anaerobic Fe(III) oxide respiration. DeltaSO3034 retained the ability to synthesize siderophores during aerobic respiration and to solubilize and respire Fe(III) at wild-type rates during anaerobic Fe(III) oxide respiration. These findings indicate that the Fe(III)-solubilizing organic ligands produced by S. oneidensis during anaerobic Fe(III) oxide respiration are not synthesized via the hydroxamate biosynthesis system and that the Fe(III)-hydroxamate reductase is not essential for respiration of Fe(III)-citrate or Fe(III)-nitrilotriacetic acid (NTA) as an anaerobic electron acceptor.

Synthesis and characterization of the solid uranium(VI) dioxo-diacetohydroxamate complex

A novel dry synthesis for the uranium(VI) dioxo-diacetohydroxamate (UAHA) complex has been developed. The complex was generated in >80% yield by mechanically grinding solid uranyl acetate dihydrate (UAc) with solid acetohydroxamic acid in stoichiometric amounts. The resulting UO2(AHA)2·4H2O solid is purified by washing with acetone. The stoichiometry was confirmedviacolorimetric assays for U(VI) and AHA. The analogous ferric trisacetohydroxamate complex (FeAHA) was synthesized for comparison. A novel dry synthesis for the uranium(VI) dioxo-diacetohydroxamate (UAHA) complex has been developed. The complex was generated in >80% yield by mechanically grinding solid uranyl acetate dihydrate (UAc) with solid acetohydroxamic acid in stoichiometric amounts. The resulting UO2(AHA)2·4H2O solid is purified by washing with acetone. The stoichiometry was confirmedviacolorimetric assays for U(VI) and AHA. The analogous ferric trisacetohydroxamate complex (FeAHA) was synthesized for comparison.

The UAHA solid was extensively characterized by ultraviolet-visible (UV-vis), Fourier-transform infrared (FT-IR), and extended X-ray absorption fine structure (EXAFS) spectroscopies. The compound did not fluoresce after laser excitation. Proton nuclear magnetic resonance (NMR) spectra were obtained of the complex in D2O, acidified acetonitrile-d3, and DMSO-d6. The solubility was determined over a range of solvents. It was determined that in the purified solid, two bidentate AHA molecules bind to uranylviathe carbonyl and hydroxamate oxygen atoms, a structure analogous to known ferric, nickel, and lanthanum AHA complexes. In an acidic environment, binding is monodentate through the hydroxamate oxygen. And in aqueous solution, the UAHA complex assumes both binding moieties, depending on the pH. This pH-dependent speciation change is demonstrated for the first time.

The easy synthesis and purification of UAHA enables researchers to strictly control reaction conditions; to eliminate interfering salts and water; and to study the complex in the solid-phase.

Toward Iron Sensors: Bioinspired Tripods Based on Fluorescent Phenol-oxazoline Coordination Sites

In the quest for fast throughput metal biosensors, it would be of interest to prepare fluorophoric ligands with surface-adhesive moieties. Biomimetic analogues to microbial siderophores possessing such ligands offer attractive model compounds and new opportunities to meet this challenge. The design, synthesis, and physicochemical characterization of biomimetic analogues of microbial siderophores from Paracoccus denitrificans and from the Vibrio genus are described. The (4S,5S)-2-(2-hydroxyphenyl)-5-methyl-4,5-dihydro-1,3-oxazole-4-carbonyl group (La), noted here as an HPO unit, was selected for its potential dual properties, serving as a selective iron(III) binder and simultaneously as a fluorophore. Three tripodal symmetric analogues cis-Lb, cis-Lc, and trans-Lc, which mainly differ in the length of the spacers between the central carbon anchor and the ligating sites, were synthesized. These ferric-carriers were built from a tetrahedral carbon as an anchor, symmetrically extended by three converging iron-binding chains, each bearing a terminal HPO. The fourth chain could contain a surface-adhesive function (Lc). A combination of absorption and emission spectrophotometry, potentiometry, electrospray mass spectrometry, and electrochemistry was used to fully characterize the corresponding ferric complexes and to determine their stability. The quenching mechanism is consistent with an intramolecular static process and is more efficient for the analogue with longer arms. Detection limits in the low nanogram per milliliter range, comparable with the best chemosensors based on natural peptide siderophores, have been determined. These results clearly demonstrate that these tris(phenol-oxazoline) ligands in a tripodal arrangement firmly bind iron(III). Due to their fluorescent properties, the coordination event can be easily monitored, while the fourth arm is available for surface-adhesive moieties. The tripodal system is therefore an ideal candidate for integration with solid-state materials for the development of chip-based devices and analytical methodologies.

Ternary Complex Formation Facilitates a Redox Mechanism for Iron Release from a Siderophore

While the naturally occurring reducing agents glutathione (GSH) and ascorbate (H2A) alone are ineffective at reducing iron(III) sequestered by the siderophore ferrioxamine B, the addition of an iron(II) chelator, sulfonated bathophenanthroline (BPDS), facilitates reduction by either reducing agent. A mechanism is described in which a ternary complex is formed between ferrioxamine B and BPDS in a rapidly established pre-equilibrium step, which is followed by rate limiting reduction of the ternary complex by glutathione or ascorbate. Spectral, thermodynamic, and kinetic evidence are given for ternary complex formation. Ascorbate was found to be slightly more efficient at reducing the ternary complex than glutathione (k4=2.1 x 10(-3) M(-1) s(-1) and k4=6.3 x 10(-4) M(-1) s(-1), respectively) at pH 7. Reduction is followed by a rapid ligand exchange step where iron is released from ferrioxamine B to form tris-(BPDS)iron(II). The implications of these results for siderophore mediated iron transport and release are discussed.